In an active sludge method for biologically processing polluted water such as sewage, excrement, or industrial wastewater, and an aerobic valuable product producing process that utilizes microorganisms, it is important to efficiently supply oxygen to the active sludge or the microbial culture solution so as to improve the efficiency of the entire processes, reduce costs and save energy. For example, a fine bubble diffusing tube, as shown in FIG. 18, has been known as such a supply means for oxygen.
In this fine bubble diffusing tube, an elastic sheet 2 with fine slits formed therein is disposed so as to cover the outer circumference of a cylindrical supporting tube 1, and each of the ends thereof is sealed with a tightening metal member 3. The gap between the supporting tube 1 and the elastic sheet 2 on one of the ends of the supporting tube 1 is allowed to communicate with a gas supply unit 4 through a through hole 5. That is, air, supplied from the gas supply unit 4, is allowed to pass through the through hole 5, and then enters the gap between the supporting tube 1 and the elastic sheet 2 so that the elastic sheet 2 is expanded. The expanded elastic sheet 2 causes the fine slits to open, and the supplied gas forms fine bubbles, and the fine bubbles are discharged into activated sludge or a microbial culture solution. Upon stoppage of the air supply, the elastic sheet 2 is contracted to cause the fine slits to close so that, when no fine bubbles are discharged, the activated sludge or the microbial culture solution is prevented from flowing into the diffusing tube through the fine slits, thereby making it possible to prevent clogging of the fine slits and stain inside the diffusing tube.
Moreover, as the fine bubble diffusing device using such a fine bubble diffusing tube, for example, a device, disclosed by Patent Document 1, has been known. As shown in FIG. 19, this fine bubble diffusing device directs air supplied from a main gas supply pipe 10 to a plurality of branch pipes 9 disposed on the two sides thereof, and the resulting air is diffused and discharged through the fine slits of the fine bubble diffusing tube 6 placed so as to communicate with the branch pipes 9.
However, in the above-mentioned fine bubble diffusing tube and fine bubble diffusing device, in the case when the length of the diffusing tube in the longitudinal direction (direction along the center axis of the cylindrical supporting tube) exceeds a predetermined length (1000 mm), although sufficient bubbles are generated from the fine slits located at a position (base side) close to the air supply unit, the amount of bubbles to be generated from the fine slits located at a position (tip side) far away from the air supply unit becomes smaller, resulting in a problem in that the amount of bubbles to be generated is made extremely uneven.
On the other hand, a submerged membrane separation apparatus that is immersed and placed in a treatment tank 8, as shown in FIG. 20, has been conventionally known as a water treatment device that carries out a filtration treatment on polluted water, such as sewage, excrement, or industrial wastewater, by using a membrane. In FIG. 20, the submerged membrane separation apparatus is immersed in a treated liquid stored in the treatment tank 8. A membrane module 23 in which a plurality of flat plate-shaped filtration membranes 22 are disposed side by side so as to be in parallel with the membrane faces is provided with a permeated water outlet 26, and an effluent piping 24 and a suction pump 25 are allowed to communicate with the permeated water outlet 26.
A treated liquid supply pipe 21 is opened above the treatment tank 8. Upon activation of the suction pump 25 serving as a driving force for filtration, the treated liquid inside the treatment tank is filtrated through the flat plate-shaped filtration membranes 22 disposed inside the membrane module 23 so that the filtrate is taken out of the system through the permeated water outlet 26 and the effluent piping 24.
The diffusing tube 6 is placed below the membrane module 23, and during a filtration operation, air supplied from the blower 7 is sent to the diffusing tube 6 through the gas supply pipe 10 and the branch pipe 9 so that the air is discharged into the treatment tank (aeration tank) 8 from the diffusing pores of the diffusing tube. An upward-moving stream of a gas-liquid mixture is generated by the air lift effect of the discharged air and this upward-moving stream of the gas-liquid mixture and bubbles act as cleaning flows on the surfaces of the filtration membranes, so that the adhesion or deposition of a cake layer onto the membrane surfaces is suppressed for a stable filtration operation (see Patent Document 2).
Relatively coarse bubbles are effective in increasing the cleaning flow effect on the membrane surfaces, and therefore, coarse bubble-generating diffusing tubes have been used. It is also proposed that fine bubble-generating diffusing tubes should be used to reduce the amount of the diffused gas. Even in such a case, the fine bubble diffusing tubes are used in combination with coarse bubble diffusing tubes so that coarse bubbles can act on the membrane surfaces (see Patent Documents 3 and 4). In such an apparatus, diffusing tubes having small diffusing holes or membrane type diffusing device plates are used as the fine bubble diffusing tubes, and such diffusing devices are placed at a predetermined location under the submerged membrane separation apparatus.
In general, fine bubble diffusing tubes are also used in a diffusing device system for supplying oxygen to microorganisms in an activated sludge liquid in a treatment tank. For example, as shown in FIG. 20, as the fine bubble diffusing tubes for the activated sludge treatment, known fine bubble diffusing tubes are so configured that air supplied from a single main gas-supply pipe 10 is guided to a plurality of branch pipes 6 placed on both sides of the pipe and diffused from fine diffusing holes formed in the surfaces of the branch pipes (see Patent Document 1). When the fine bubble diffusing tubes have such a structure, fine bubbles are not diffused from the central region where the main gas-supply pipe 10 is located; however, as long as oxygen is supplied to an activated sludge liquid, no problem is raised. However, when such a diffusing device is placed under a submerged membrane separation apparatus as shown in FIG. 20, the air lift effect is hardly produced at the central portion of the diffusing device where no fine bubbles are diffused, with the result that it is not possible to obtain the cleaning flow effect on the membrane surfaces. As a result, a problem occurs in which membrane surface cleaning is insufficient in the central portion of the submerged membrane separation apparatus so that the filtration function of the separation membrane is lowered.
Moreover, as the diffusing device to be placed below the submerged membrane separation apparatus, a device disclosed in Patent Document 5 has been known. This diffusing device has a structure in which, although the two ends of the diffusing tube are connected to gas supply pipes, the gas supply can be carried out only from one side, with no gas supply being available from both of the sides. The gas supply only from one side makes it difficult to generate bubbles from the entire diffusing tube, and consequently makes it also difficult to exert cleaning flow effects on the entire separation film on the upper portion of the diffusing tube. Moreover, the device described in Patent Document 5 also has a structure in which, for example, a connecting process using a thread opening is carried out on each of the connecting portions Y on the two ends; however, the connecting process by the use of the thread opening is carried out by rotating the gas supply unit 4 or the branch pipe 9 so that it is shifted in parallel with the longitudinal direction α of the supporting tube 1 so as to be thread-engaged. In the case when this thread-engaging process is used in both of the ends, if a single fine bubble diffusing tube is attached to the branch pipe 9 at each of the two ends, the connecting process is carried out without causing any problems; however, this structure makes it difficult to connect two or more fine bubble diffusing tubes arranged in parallel with one after another at their two ends.    Patent Document 1: JP-A No. 2005-081203    Patent Document 2: JP-A No. 10-296252    Patent Document 3: JP-A No. 2001-212587    Patent Document 4: JP-A No. 2002-224685    Patent Document 5: JP-A No. 2006-263716